Many television sets or television signal converters can be enabled to insert a display such as time, channel number, etc. (referred to herein as an insert signal) into a received video picture. The synchronization of such inserted signals with the sync pulses of the received video picture must be carefully controlled. Circuits to perform this function normally operate satisactorily in the presence of a received composite video signal. However where a composite video signal is absent, such as on an unused channel or if the video signal transmitter has gone off the air, or on a pay TV channel containing video with suppressed or absent sync pulses or which is otherwise scrambled, there is no incoming sync signal to control the placement of the inserted time, channel number, etc. on the screen. The result is typically an insert signal appearing on the TV screen which contains objectionable amounts of jitter.
Where a cable converter interfaces a television set with e.g. a CATV system, which converter is under control from the CATV head end system, sometimes insert digital signals or insert control signals are transmitted on a spare channel, and the insert signals are intended to be viewed alternatively on a channel on which there is or is no incoming video signal. In the latter case it is very important that the displayed insert (foreground) signal should not contain jitter. It is also desirable in such cases that a stable colour background should be visible to the television viewer.